hil: express a board's always-on defines as a variant, dropping build.args

The roster had two ways to pass a cmake -D to a board's build: `build.args`,
applied to every variant, and `variant[].defines`, applied to one. They did the
same thing, and only metro_m4_express used the first - for MAX3421_HOST=1, which
is what makes it the one rig board that compiles hcd_max3421.c.

A board whose define is always on now carries a single variant named after itself,
which is exactly the shape `board.get('variant') or [{'name': name, 'flags': ''}]`
already synthesises everywhere - so the build dir, the HIL report row and the
variant-boundary handling are unchanged. raspberry_pi_pico has used that shape
for its flags all along.

Removes the BuildCfg type and the parallel code path from all four consumers:
hil_test.build_board, hil_pool_check's two builders, hil_ci_set_matrix and
ci_select.board_options.

Verified: the hil-build matrix entry is byte-identical
(`-b metro_m4_express -DMAX3421_HOST=1`), hil_test's build command is unchanged,
ci_select still selects the board for a max3421 diff with MAX3421_HOST in its
options, and a real build of dual/host_info_to_device_cdc and host/cdc_msc_hid on
that board still compiles hcd_max3421.c.

1 files changed, 6 insertions, 4 deletions

diff --git a/tools/ci_select.py b/tools/ci_select.py
index cd63899c1..ced3bbbc0 100755
--- a/tools/ci_select.py
+++ b/tools/ci_select.py
@@ -188,10 +188,12 @@ def bsp_board_options(board_name: str, repo_root: str) -> frozenset:
 
 
 def board_options(board: dict, repo_root: str) -> set:
-    """Build options a board has truthy: the roster entry's build.args plus each
-    variant's defines (NAME=VALUE) and raw CFLAGS (-DNAME=VALUE), plus whatever its
-    own board.cmake sets (a board can enable a gated port without the roster saying so)."""
-    toks = list(board.get('build', {}).get('args', []))
+    """Build options a board has truthy: each variant's defines (NAME=VALUE) and raw
+    CFLAGS (-DNAME=VALUE), plus whatever its own board.cmake sets (a board can enable a
+    gated port without the roster saying so). A board whose option is always on carries
+    a single variant named after itself - metro_m4_express and MAX3421_HOST=1, which is
+    what makes it the one rig board that compiles hcd_max3421.c."""
+    toks = []
     for v in board.get('variant', []):
         toks += list(v.get('defines', []))
         toks += v.get('flags', '').split()